LibGUI: Add Widget focus policies

Every widget now has a GUI::FocusPolicy that determines how it can
receive focus:

- NoFocus: The widget is not focusable (default)
- TabFocus: The widget can be focused using the tab key.
- ClickFocus: The widget can be focused by clicking on it.
- StrongFocus: Both of the above.

For widgets that have a focus proxy, getting/setting the focus policy
will affect the proxy instead.

Libraries/LibGUI/AbstractButton.h

@@ -53,7 +53,6 @@ public:
     bool is_being_pressed() const { return m_being_pressed; }
 
     virtual void click(unsigned modifiers = 0) = 0;
-    virtual bool accepts_focus() const override { return true; }
     virtual bool is_uncheckable() const { return true; }
 
     int auto_repeat_interval() const { return m_auto_repeat_interval; }